Device for stable subsea electric power transmission to run subsea high speed motors or other subsea loads

ABSTRACT

The invention provides a device for operative connection between a subsea step out cable far end and subsea loads such as pumps, compressors and control systems, distinctive in that the device is a rotating frequency stepper device, more specifically a rotating step up or step down device, and it comprises: a motor and a generator operatively connected so that the motor drives the generator, at least one gas and/or liquid filled vessel into which at least one of the motor and generator are arranged, and the step out length is long, which means long enough to cause problems due to the Ferranti effect at frequency and power levels feasible for subsea pump and compressor motors, and where the device via the step out cable receives input electrical power at a low enough frequency to have stable transmission and the device, operatively connected to the subsea motor, delivers an output electrical frequency, amperage and voltage feasible for operation of the connected motors. System for pressure boosting of hydrocarbon fluid or other fluid subsea, comprising the device.

FIELD OF THE INVENTION

The present invention relates to equipment for subsea production ofpetroleum, particularly equipment located far away from dry topside oronshore locations. More specifically, the invention relates to equipmentfor electric power transmission to subsea loads that can be located faraway from surface platforms or shore and require high powertransmission. Said loads are typically motors for pumps and compressorswhich require control of rotational speed by control of the electricfrequency.

The invention come to grips with the problems caused by the Ferrantieffect and the skin effect, thereby opening up for longer subsea stepout lengths than previously achievable.

BACKGROUND OF THE INVENTION AND PRIOR ART

Over the last decades global energy consumption has increasedexponentially and no end can be seen for the increased demand. Whereasexploitation of fossil fuels was previously focused on onshore fields,the limited amount of oil started serious efforts to find and exploitoffshore gas and oil fields. Presently the state of the art forproduction from offshore fields is by use of fixed or floating mannedplatforms, and by tie-in of subsea production templates with subseawells to these platforms. In some cases production is routed directly toan onshore receiving facility without a platform. In order to maintain asufficiently high production from subsea satellites to a centralplatform or directly to shore, pressure boosting can be provided byusing a multiphase pump or by separation followed by pumping andcompression. Pumps have also been installed at seabed for directseawater injection into the reservoir for pressure support for enhancedoil production.

-   -   There are several advantages that motivate for subsea location        of pumps and compressor stations compared to location on        platforms:    -   Safety for people by not working and living on platform and not        being transported by helicopters to and from    -   No risk of fire and explosion    -   No risk for blow-out from production risers up from seabed to        platform and from platform to seabed    -   Security against sabotage    -   Cost saving both for capital and operation, i.e. reduced        production cost for oil and gas    -   Increased production because the suction effect of compressors        and pumps is closer to the wellheads    -   The equipment has stable ambient conditions, i.e. almost        constant, cold temperature and almost constant, low flow        seawater current velocity around the equipment and no waves,        while the temperature at platforms can vary from e.g. −20° C. to        +30° C. and the wind velocity can be at hurricane strength        combined with extremely high waves.    -   The cold seawater can be utilized for cooling of motors and        other electric and electronic equipment and process fluids    -   No visual pollution    -   Considerably lower weight and thereby lower material and energy        amount for fabrication of a subsea plant    -   Lower carbon dioxide, i.e. climate gas emission for fabrication        due to less material amount    -   Less carbondioxide emissions during operation due to elimination        of helicopter transport and operation of platform    -   Less carbondioxide emission compared to platforms due to        electric motors for running compressors and pumps and supply of        electric power from shore or platform    -   Less energy consumption and climate gas emission per weight unit        of oil and gas

The disadvantage for subsea compressors per 2010 is that none has beeninstalled and operated subsea, i.e. the technology is not proven.However, this is just a question of time, and the first subseacompressor station will probably be in operation in 2015 or earlier dueto the strong motivation for this application.

Subsea pressure boosting is a recent technology. Subsea pressureboosting requiring a significant subsea step out length is a very recenttechnology using modern equipment and facing problems that are not metor is irrelevant elsewhere.

State of the art technology is defined in patent publication WO2009/015670 prescribing use of a first converter arrangement in the nearend, the topsides or onshore end, of a subsea step out cable and asecond converter arrangement in the far end, the subsea remote end, ofthe subsea step out cable. A variable speed drive, VSD, is prescribed ineither end of the step out cable. Subsea variable speed drives (VSD) forelectric motors is also called variable frequency drive (VFD) orAdjustable Frequency Drive (AFD) or frequency converters or justconverters and they represents state of the art technology. Neither inWO 2009/015670 or other publications is the Ferranti effect mentioned,nor is any problems associated with subsea VSDs discussed or indicated.

So far only a few subsea pumps and no subsea compressors are inoperation. Subsea compression stations are however being developed andthe first expected to be installed and in operation within some fewyears. Currently, subsea pumps and compressors are all driven byasynchronous motors. The step-out distance of installed pumps is notmore that about 30 km from platform or shore and so far the depths arenot below 1800 m. It is known that serious studies and projects areconducted by the oil industry aiming at installation of compressors at astep-out distance in the range of 40 to 150 km and at water depth downto 3000 m or more.

A realistic motor power is from about 200 kW for small pumps and up to15 MW for compressors and in the future even larger motors can beforeseen. Subsea motors that are presently installed are supplied withpower via AC (alternating current) cables from the location of the powersupply, i.e. platform or shore, and in case of several motors each motorhas its own cable and frequency converter (Variable Speed Drive, VSD) atthe near end of the cable in order to control the speed of eachindividual motor at the far end of the cable, ref. FIG. 1 and Table 2.

In the context of this patent description near end means the end of thepower transmission near to the power supply. In subsea applications thisis topsides platform location or onshore. Correspondingly, the far endrefers to the other end of the transmission line close to the powerloads, typically motor loads. Far end is not necessarily restricted tothe high-voltage end of the transmission line. The term can be extendedto busses or terminals of lower voltage which are part of the far endstation such as e.g. a common subsea bus on the low-voltage side of asubsea transformer.

Compressors and pumps are often operated at maximum speeds between 4000to 14000 rpm and 2000 to 5000 rpm, respectively. Thus the drivingelectrical motor has to have a rated speed in the order 2000 to 14000rpm when using modern high speed motors without a gearbox between themotor and the pump or compressor. This mechanical speed corresponds toan electrical frequency range for the feeding drive of about 30 to 230Hz for the example of a two-pole motor. Motors with more pole pairswould allow for lower maximum mechanical speed for the same electricalfrequencies.

FIG. 1 illustrates the only solution so far used for transmission ofelectric power to installed pumps, in some cases without transformersbetween VSD and subsea motors, and this is referred to as Firstsolution. This solution with one transmission cable per motor has thedisadvantage of becoming expensive for long step-out; say more than 50km, due to high cable cost.

A serious technical obstacle against this solution is that at a certainsubsea step-out length, the transmission of electric power from a nearend power source to a far end distant motor is not feasible because thetransmission system will become electrically unstable and inoperable dueto the Ferranti effect that later will be described. The innovation willresolve this problem of instability.

FIG. 2 illustrates a solution that has been proposed for transmission ofelectric power to several loads at long step-out, Solution Two. Thissolution with one common transmission cable and a subsea powerdistribution system including one subsea VSD (Variable Speed Drive) permotor, will considerably reduce the cable cost for transmission, andalso prevent the problem of electric instability by limiting thefrequency of the current in the transmission cable to say 50-10 Hz, andthe skin effect is also acceptable for such frequencies. The frequencyis then increased by a VSD to suit the speed of the motor connected tothe VSD. The Second Solution has however also disadvantages. These areexpensive VSDs which are not proven for subsea use, and because suchVSDs are composed of many electric and electronic components included acontrol system, they are susceptible to contribute to an increasedfailure rate of the electric transmission and subsea distributionsystem.

In the following will be described the inherent electrical problems ofthe existing First Solution (FIG. 1), with one motor at the far end of along cable, and a Third Solution illustrated in FIG. 3 with severalmotors at the far end of a common long transmission and a common VSD atthe near end.

For a long step-out distance from the power supply to the load, in theorder of 50 km and above, the influence of the subsea cable is so strongthat such a system has not been built yet for a limited load such as asingle motor. The line inductance and resistance involve a large voltagedrop from the power supply to the load. It is known that such a voltagedrop is self-amplifying and can result in zero voltage at the far end.The longer the step-out distance the higher the transmission voltage hasto be in order to reduce the voltage drop along the transmission line.However, a cable has a high capacitance and a long AC (alternatingcurrent) cable will exhibit significant so-called Ferranti effect. TheFerranti effect is a known phenomenon where the capacitive chargingcurrent of the line or cable increases with the line length and thevoltage level. At a step-out length of 100 km the charging current in acable can be higher than the load current, which makes it difficult tojustify such an ineffective transmission system. A more critical resultis that the sudden no-load voltage will be about 50% higher than thenear end supply. Such highvoltage would destroy the cable and the farend transformer and connections. At a sudden load drop the far endvoltage will jump to this high level. In addition there will be atransient peak of e.g. 50% giving like 100% in total, see Table 1 belowwhere values marked with fat italic letters are above the voltage classmargin of the insulation.

Today's systems with step-out distances in the order 30 km have not thisproblem, because the subsea step-out length and electric load incombination is still feasible.

TABLE 1 Voltage rise at load trips due to Ferranti effect in differentsystems Max. Far end transient Far transmission frequency f_(max) andStep- Source Full-load voltage peak u_(p) after end shaft power motorspeed ω_(max) out length Standard cable voltage at near end U andno-load voltage U full-load trip Pump 60 Hz  40 km 95 mm² 20 kV 18.3 kV20.9 kV 2.5 MW (3600 rpm) 30(36) kV 20.2 kV First Solution Compressor180 Hz  40 km 150 mm² 32 kV 29.2 kV

7.5 MW (10800 rpm) 30(36) kV 34.8 kV First solutions Pump 60 Hz 100 km150 mm² 26 kV 23.6 kV 28.9 kV 2.5 MW (3600 rpm) 30(36) kV 27.5 kV FirstSolution Compressor 180 Hz 100 km 150 mm² 28.5 kV   28.8 kV

7.5 MW (10800 rpm) 30(36) kV

First Solution Three 180 Hz 100 km 400 mm² 45.6 kV   45.6 kV

compressors and Compressor: 45(54) kV

three pumps. 10800 rpm Total 30 MW Pump: Third solution 5400 rpm

The Ferranti effect and skin effect—some considerations:

The Ferranti effect is a rise in voltage occurring at the far end of along transmission line, relative to the voltage at the near end, whichoccurs when the line is charged but there is a very light load or theload is disconnected.

This effect is due to the voltage drop across the line inductance (dueto charging current) being in phase with the sending end voltages.Therefore both capacitance and inductance are responsible for producingthis phenomenon.

The Ferranti effect will be more pronounced the longer the line and thehigher the voltage applied. The relative voltage rise is proportional tothe square of the line length.

Due to high capacitance, the Ferranti effect is much more pronounced inunderground and subsea cables, even in short lengths, compared to airsuspended transmission lines.

A proposed equation to determine the Ferranti effect for a given systemis:v _(f) =v _(n)(1+ω×C×L×I ²)Where:v_(f)=far end voltagev_(n)=near end voltageω=2×3.14×ff=frequencyC=line capacitanceL=line inductanceI=line lengthI²=line length square

In the literature can also be found other expressions for the Ferrantieffect, but in any cases it is agreed that the effect increases withtransmission frequency, cable capacitance, length of cable and voltage.

From the above equation can be concluded that the Ferranti effect of along line can be compensated by a suitable reduction of the electricfrequency. This is the reason for the Second Solution with subsea VSD.The transmission frequency can e.g. be the normal European frequency of50 Hz.

Another benefit with low transmission frequency is a strong reduction ofthe electrical skin effect of the transmission cable, i.e. betterutilization of the cross section area of the cable. In practicetransmission of high frequency electricity, say 100 Hz or more over longdistances, say 100 km or more, will become prohibitive due to the skineffect and the corresponding high resistance of the cable.

The influence of Ferranti effect and skin effect has of course to becalculated from case to case to assess whether they are acceptable ornot for transmission at a given frequency. A demand exists for providingsubsea electric power transmission systems that are beneficial withrespect to the above mentioned problems.

FIGURES

The invention is illustrated with figures, of which

FIGS. 1-3 illustrate prior art embodiments, and

FIGS. 4-7 illustrate embodiments of the present invention.

SUMMARY OF THE INVENTION

The invention provides a device for operative connection between asubsea step out cable far end and subsea loads such as pumps,compressors and control systems, distinctive in that the device is arotating frequency stepper device, more specifically a rotating step upor step down device, and it comprises:

-   -   a motor and a generator operatively connected so that the motor        drives the generator,    -   at least one gas and/or liquid filled vessel into which at least        one of the motor and generator are arranged, and    -   the step out length is long, which means long enough to cause        problems due to the Ferranti effect at frequency and power        levels feasible for subsea pump and compressor motors, and where        the device via the step out cable receives input electrical        power at a low enough frequency to have stable transmission and        the device, operatively connected to the subsea motor, delivers        an output electrical frequency, amperage and voltage feasible        for operation of the connected motors.

The device is preferably a passive frequency step up device or frequencystep down device, having no means for active control or adjustment onsite subsea, and it comprises: a rotatable shaft having a motorarranged; a generator arranged on the motor shaft or on a differentshaft operatively connected to the motor shaft; a pressure vessel intowhich the motor, generator and shafts are arranged; a gas and/or liquidfilling the pressure vessel, at least one electric penetrator and apressure compensator if the vessel is filled with liquid that is to bepressure compensated to the ambient seawater pressure. The frequencystep down can be all the way down to 0 Hz, the frequency step up can beup to the operating frequency of the connected loads.

More preferably the device is a Subsea Rotating Frequency Step-up Device(SRFSD) comprising an electric motor coupled to a generator for subsealocation at a far end of a subsea step out cable connected to at leastone power source at the step out cable near end at a dry locationonshore or topsides, and the step out length is long, which means longenough to cause problems due to the Ferranti effect at frequency andpower levels feasible for subsea pump and compressor motors, and wherethe device via the step out cable receives input electrical power at alow enough frequency to have stable transmission and the device,operatively connected to the subsea motor, delivers an output electricalfrequency, amperage and voltage feasible for operation of the connectedmotors and the device is installed in a pressure vessel or housing thatis filled with liquid or gas.

Most preferably the device comprises an electric motor and an electricgenerator having a common shaft, the pole number of the generator is amultiple of the pole number of the motor. Alternatively the devicecomprises one of: a mechanical gear, a fluid-dynamic or hydraulic gear,a mechanical fluid dynamic gear or a magnetic gear.

No earlier subsea pressure boosting systems has taken into considerationthe Ferranti effect. The earlier system version with a subsea VSD cantherefore be useless for many applications since the insulation of thestep out cable can be damaged by uncontrollable high voltage at the farend due to the Ferranti effect. The feature a “passive electricfrequency step up or step down (or stepper) device”, relevant for someembodiments, means that the device shall not and can not be adjusted onsite during operation or any time during the service life of the system,the device is a passive slave unit, namely a passive frequency step updevice or a passive frequency step down device, contrary to a subseaVSD. A subsea VSD is very complex, large and expensive, it is typicallyabout 12 m high, 3 m in diameter and weights about 200 tons. The passivedevice will to the contrary be much smaller and simpler, being typicallyabout 6 m long and 2-3 m in diameter, weighting about 50 ton. Thereliability of the device is estimated to be several times better thanfor a subsea VSD. This is because a subsea VSD is very complex, and eventhough all components are of top quality the large number of componentsand the complexity results in a reduced reliability in practice. Thecost of the device or a system of the invention will be significantlyreduced compared to the state of the art systems having a subsea VSD.The term other loads comprises power to control systems and other loadsnot necessarily related to pressure boosting.

The operation frequency of the step out cable must be considered takinginto account the Ferranti effect and the electrical losses. Theinsulation is a key element. Most preferably, the dimensions ofconductors and insulation, and choice of operation frequency, are sothat at the far end of the cable, the Ferranti effect, at its maximumduring operation, increases the voltage just as much as the electricallosses, hence overvoltage at the far end due to the Ferranti effect isavoided and the cable design is simplified. The guidance provided inthis document, combined with good engineering practice, is assumed to besufficient for proper step out cable design, including choice ofoperation frequency: The solution should be found in each case. Thedevice of the invention is then designed in order to transform theoperation frequency of the step up cable to the operation frequency ofthe subsea loads, i.e. subsea compressors or pumps, or morespecifically, the motors of the subsea compressors or pumps.

Further embodiments and features are defined in the dependent claims.The features described or illustrated in this document can be includedin the device of the invention in any operative combination, and eachsuch combination is an embodiment of the invention. The motivation forsuch combinations is based upon what is described or illustrated or thecombinations are obvious for persons skilled in the art after havingstudied this document thoroughly.

The input and output electrical frequency of the device will bedifferent. The difference will be at a fixed ratio for passive devices.The input frequency, the operation frequency of the step out cable, willbe in the range 0.1-150 Hz, such as 2-60 or 4-50 Hz or 5-40 Hz, whilstthe output frequency will be in the range 0.1-350 Hz, such as 30-300 Hz,50-250 Hz or 50-200 Hz. The output frequency can also be 0, i.e. directcurrent (DC) by using a DC generator in the motor-generator set. Thesubsea device can be arranged in one or several housings, as one orseveral elements, however, all parts of it must withstand the harshsubsea environment without failure. With the present invention, the longterm cost and reliability of said device, and associated systems,improve significantly over what is currently achievable with for examplesubsea solid state variable speed drives.

The invention also provides a system for pressure boosting ofhydrocarbon fluid or other fluid subsea, comprising

-   -   a subsea step out cable, connected to a an electric AC power        source at a near end, the length of the subsea step out cable is        too long for stable operation at frequency and power level        feasible for subsea pressure boosting equipment,    -   subsea motors for pumps or compressors operatively connected to        a far end of the subsea step out cable,    -   distinctive in that the system further comprises: a rotating        motor-generator frequency step up device arranged between the        subsea step out cable and subsea pumps or compressors.

Preferably, the device of the system has no means for active control oradjustment on site subsea, and it comprises:

-   -   a generator arranged on a motor shaft,    -   a vessel into which the motor, generator and shaft are arranged,    -   a liquid filling the vessel,    -   a pressure compensator, and    -   at least one electric penetrator.

In addition, the invention provides use of a subsea rotating stepperdevice of the invention for transforming the electrical powercharacteristics of a subsea step out cable to an electric powercharacteristic feasible for operation of connected subsea equipment, asystem with at least one subsea stepper device of the invention arrangedin the far end of a subsea step out cable, and a method of operatingsaid system, by control adjustments only for system items at drytopsides or onshore locations, such as by a topsides VSD. Either one ofthe device, the system, the method or the use of the invention, maycomprise any features or steps as herein described or illustrated, inany operative combination, each such operative combination is anembodiment of the invention.

The Embodiment of the Invention with Frequency Step-Up to Run AC Motors

An embodiment of the invention, the Fourth Solution is shown in FIGS. 4and 5. The main feature of the embodiment is introduction of a subseafrequency step up or step down device, in the illustrated embodiment afrequency step-up device (FSD) located subsea at the far end of thetransmission cable and at a short distance to the motors that runs thecompressors and pumps. Short distance means in this context near enoughto keep acceptable the ohmic resistance drop and thereby power lossbetween the generator/FSD and the motors, and it also means short enoughto avoid problems caused by Ferranti effect and instability. It isimportant to note that the subsea FSDs are not directly controlling thefrequency to suit the operational speed of motors by having a localcontrol system that adjusts the speed according to needs. The variationof speed according to steady state production need, start and stop andramping speed down and up, is done by the near end surface (topsides onplatform or onshore) located VSD or by other means far from the subseaFSDs. The FSDs are simply slaves of the VSD and their purpose is onlystepping-up the transmission frequency given by the VSD by somemultiple.

This step-up is easiest obtained by using a subsea electric motor whichshaft is coupled to a subsea electric generator and both machinesrunning with same speed, i.e. a subsea rotating FSD (RFSD). Any type ofcoupling (e.g. flexible, rigid, common shaft of motor and generator,hydraulic, fluid coupling) can be used that gives the same speed ofmotor and generator. The motor should preferably have 2-poles to keepthe transmission frequency as low as possible, while the generator'snumber of poles will be chosen according to the need for step-up from atransmission frequency that is low enough to not give the abovedescribed problems caused by Ferranti effect, instability and highresistance due to skin effect with corresponding unacceptable voltagedrop; i.e. within a “problem free frequency range”.

By having a 2-pole motor and a 4-pole generator the step-up ratio willbe 2:1, a 6-pole generator will give a ratio of 3:1 and an 8-polegenerator 4:1 and so on dependent of the number of poles of thegenerator. This means that if the frequency from a surface VSD is in therange of 50 the subsea frequency from the subsea RFSD device will be inthe range of 100 Hz corresponding to a revolutionary speed of 2-polemotors from 6000 rpm. If using an 8-pole generator the correspondingstepped-up frequency will be in the range of 200 and the speed of a2-pole motor 12000 rpm. These examples clearly demonstrate that theinvention can supply any needed frequency for realistic motor speeds bya correct combination of poles of motor and generator of the rotatingsubsea RFSD and at a problem free transmission frequency.

Generally the step-up ratio can be expressed:fs-u=n×ft, whereft: transmission frequency, Hzfs-u: stepped-up frequency=input frequency to motors, Hzn: multiple 2, 3, 4 and so on dependent of number of poles of thegenerator compared to the motor

The problem free frequency range must be calculated from case to case.For step-out distances of up to say 150 km a transmission frequency ofup to say 75 Hz could be within the problem free range which will give a2-pole compressor motor speed of 2×7×60=9000 rpm if the step-up ratio is2:1 (2-pole motor and 4-pole generator). If 75 Hz is found to be to highto be problem free, a step-up ratio of 3:1 (2-pole motor and 6-polegenerator) can be applied, which for the given example will reduce thetransmission frequency to maximum of 50 Hz. The transmission frequencywill not stay constant over the whole production period of the oil orgas field, but have to be adjusted up over time as the pressure at thewellheads decreases. For a given case the transmission power from thenear end could be 33.3 Hz by the beginning and 50 Hz by the end ofproduction corresponding to a speed of between 6000 and 9000 rpm of a2-pole compressor motor at the far end.

By selecting the right step-up ratio by selection of poles of motor andgenerator, it will probably be possible to transmit AC power problemfree to subsea motors with a distance from the near end to the far end(step-up distance) of 300 km or more.

Use of a 2-pole motor is beneficial to keep the transmission frequencyas low as possible. If there of other reasons, e.g. torque and power,should be found favourable to use a motor with higher number of poles,it is still possible to get a desired step-up by selecting the number ofpoles of the generator correspondingly, e.g. 4-pole motor and 12-polegenerator will give a step-up ratio of 3:1.

An advantage by using low frequency and 4-poles motor is that the speedof the motor and generator will be low and so will be the correspondingfrictional losses in the motor. This opens for use of oil filled motorand generator arranged in common pressure housing,

If for instance the transmission frequency is 25 Hz and a 4-pole motoris used the rotational speed will be only 750 rpm, which will result inlow frictional losses. To achieve a frequency of 150 Hz from thegenerator, this has to be 24-pole. By varying the transmission frequencyfrom 18 to 28 Hz, the frequency from the generator will vary in therange from 108 to 168 Hz and give motor speed (2-pole) of 6480 to 10080,which could be suitable for a compressor motor.

The selection of the region of the variable transmission frequency andthe consequential necessary step-up ratio will therefore be based on alow enough frequency to have a stabile transmission for the givenstep-out distance and keep the Ferranti effect and skin effect lowcombined with a suitable number of poles and torque of the motor and thegenerator. Additionally, if oil filled motor and generator arepreferred, the speed must be kept below some limit to avoid too highfrictional losses; typically could a speed of 750 to 1500 rpm befavourable, i.e. a transmission of 25 Hz to obtain 750 Hz for 4-polemotor and 1500 rpm for 2-pole motor.

Below is given as an example a table that shows the resulting speed of asubsea compressor drive (motor) with 2-poles by using a motor-generatorset with 4-poles motor and 12-poles generator:

Transmission Output frequency Speed of 2-poles frequency, Speed of4-poles from 12-poles compressor drive, Hz motor, rpm generator, Hz rpm5 150 15 900 10 300 30 1800 20 600 60 3600 25 750 75 4500 30 900 90 540040 1200 120 7200 50 1500 150 9000 60 1800 180 10800 70 2100 210 12600 802400 240 14400

The table demonstrates that a transmission frequency range up to 50 Hzwill cover the most actual speed range for compressors.

A similar table is given below for a compressor drive with 2-poles, a6-poles motor for the motor-generator set and 24-poles generator:

Transmission Output frequency Speed of 2-poles frequency, Speed of6-poles from 24-poles compressor drive, Hz motor, rpm generator, Hz rpm1 20 4 240 5 100 20 1200 10 200 40 2400 20 400 80 4800 25 500 100 600030 600 120 7200 40 800 160 9600 50 1000 200 12000 60 1200 240 14400 701400 280 16800

In this case a transmission frequency of up to 40 Hz will be sufficient.

The above tables clearly demonstrate that the transmission frequency canbe kept low to avoid problems caused by Ferranti effect and skin effect.

Selection of compressor bundle is also a factor that helps to givefreedom in selection of transmission frequency and frequency step-upratio, i.e. a bundle can be selected, within reasonable limits, to suitan f_(s-u) resulting from an optimum transmission system.

A subsea RFSD is in principle quite simple and no control system is needbecause the stepped-up frequency will be automatically obtained as aresult of the ratio of poles of the generator relative to the poles ofthe motor of the RFSD.

Another advantage with a subsea rotating step-up device is that theoutput current and voltage will have a practically perfect sine waveform which is beneficial for the motors, i.e. no electric filter forsmoothening is needed to obtain this.

The subsea RFSD (SRFSD) also supplies inductance to the transmissionsystem, which due to the cable has a surplus of capacitance, and theSRFSD therefore reduce the need for near end electric phasecompensation.

There will be some power loss in a SRFSD, say 5%, but a subsea VSD willalso have losses, however perhaps lower.

The selection of SRFSD must of course be such that the output power ofthe generator at a given frequency is such that it corresponds to thedemand of the connected motor(s). If for instance a 2-pole compressormotor shall give 10 MW at 10000 rpm, the power output of the generatormust be accordingly plus a little additionally to cover for losses at afrequency of 167 Hz. The motor of the SRSFD must correspondingly give ashaft power of 10 MW plus some additionally to cover for losses.

Another way than having different poles of the motor and generator ofthe motor-generator set, can be to include a fixed step up gear betweenthe motor and the generator, e.g. of 3:1. If the transmission frequencyfor instance is 50 Hz, a 4-poles motor will have a speed of 1500 and thegenerator speed will be 4500 rpm with an output frequency of 150 Hz thatgives a 2-poles compressor drive a speed of 9000 rpm. A combination offixed step-up and number of generator poles can also be used to keep thenumber of poles down if favourable. If for example a step-up gear withratio 2:1 is inserted between a 4-poles motor and an 8-poles generator,the speed of the motor at 50 Hz will be 1500 rpm, the speed of thegenerator 3000 rpm and its frequency output 200 Hz and the speed of thedrive 112000 rpm. By having VSD at the near end the speed of the drivecan be adjusted to suitable values by adjusting the transmissionfrequency in the range up to 50 Hz.

In some cases can be kept a fixed transmission frequency and thereby afixed frequency from the generator and hence a fixed speed of theconnected motor, e.g. compressor, multiphase or single phase pump motor.If the motor runs a compressor, the compressor speed can for instance bekept constant at 9000 rpm, and a suitable flow capacity and pressureratio of the compressor, which will vary over time, can be adjusted byrebundling and some recirculation. This will give the simplest andlowest CAPEX of the total system, but with somewhat higher power lossesdue to periods with recirculation on the compressor. A more frequentrebundling of the compressor may also be necessary compared to variablefrequency. An optimum power transmission and compression system must bebased on calculations to establish optimum system design from case tocase.

Design of Subsea RFSD

Oil Filled Pressure Housing

The motor and generator are assembled in a common pressure housing witha suitable number of flanges with seals. Further there are severaloptions for the practical design, which are listed in the following:

The motor-generator has a suitable number of bearings.

The rotational speed of the motor-generator is low enough to keep thefrictional losses acceptable, and the common pressure housing is filledwith a suitable liquid, e.g. oil, that lubricates the bearings and alsocools motor and generator and the properties of the selected oil shouldpreferably be such that it serves as electric insulator.

Instead of oil, the housing can be water filled with water or a mix ofwater and antifreeze agent, e.g. MEG, which requires a completeelectrical insulation of the motor and generator windings.

The pressure inside the housing can be selected freely by not filling itcompletely with liquid and have a gas volume at some pressure.

A favourable solution is to fill the housing with liquid and havepressure balancing device between the ambient seawater and the internalliquid of the pressure housing. This will result in a minimum thicknessof the pressure housing and also reduce the load and requirements toflanges and seals

If the direct cooling of the motor-generator by heat flow through thepressure housing and to the sea is too low, has to be included anexternal cooling circuit with heat exchange to the ambient seawater.

The pump for the cooling circuit can favourably be coupled to themotor-generator shaft or it can be a separate pump with electric motor.

If magnetic bearings for operation in liquid are available, this couldbe an option to liquid lubricated bearings. For more details about this,reference is made to the description below for gas filled housing.

Gas Filled Housing

The pressure housing can be filled with an inert gas, e.g. dry nitrogenor dry air. The advantage of this is lower frictional losses than foroil filled, which allows higher speed of motor-generator. Additionallythe practical solution can include the following:

Liquid lubricated bearings (e.g. oil, water or water/MEG) with acirculating circuit through an external heat exchanger or only insidethe housing.

Minimum one pump for the lubricant, either driven by the motor-generatorshaft or a separate electric pump

If necessary a cooling circuit for the gas is included by having minimumone fan to circulate the gas through an external heat exchanger or onlyinside the housing.

Alternatively to liquid lubricated bearings, magnetic bearings can beused. The cooling system for the gas must then be dimensioned to alsocool the magnetic bearings.

A control system for the magnetic bearings must be included, located inthe vicinity of the motor-generator housing or inside the housing. Ifthe control system is located in a pod outside the motor-generatorhousing, penetrators through the housing wall are needed as well aswires for power and signals between the control system and the magneticbearings. If the control system is in a pod, the pod can be designed tobe separately retrievable or not.

The pressure inside the housing can be selected from in the region ofone bar and up to equal to the ambient water pressure or higher. Theadvantage of low pressure is low friction and losses. The advantage ofhigh pressure is that the heat capacity of the gas increases withpressure and therefore gives better cooling. Another advantage of highpressure is also reduced requirement to wall thickness and lower load onflanges and seals. If the pressure is selected close to equal to ambientseawater pressure, the resulting requirements to the pressure housingand flanges and seals will be similar to a liquid filled pressurebalanced vessel.

Subsea Rotating VSD

Above is mentioned use of hydraulic or fluid coupling between the motorand the generator in the motor-generator set. Such a coupling has theadvantage of giving “soft start”, i.e. the generator load on the motoris not immediate, but ramps up over some time such that a high startcurrent peak is avoided. The use of such a coupling can be furtherexpanded to make the coupling adjustable such that the speed of thegenerator can be adjusted relative to the constant motor speed. In thisway the motor-generator set can be used as a subsea variable speeddrive, i.e. subsea rotating variable speed drive (RVSD), and the topsideVSD can be omitted.

Instead of a fluid coupling can be used a mechanical gear for steppingup and down the speed of the generator, and thereby its outputfrequency.

If a variable coupling of some kind (fluid or mechanical) is used, thecontrol system for the variable coupling can be in a separate podexternally to the subsea RVSD or it can preferably be surface locatedand preferably connected to or integrated in the overall control systemfor the subsea booster station, compressor station or subsea processingplant or other system with subsea motors with variable speed.

Some Considerations

One important point of the invention is that though typically a VSD isused at near end, it is not important to be able to quickly adjust thefrequency of the motor loads. The motor speed is slowly adjusted overyears while the reservoir is produced and the field pressure graduallydecreases thus requiring increased power, i.e. motor speed. This factallows for e.g. temporarily ramping down running motors in order toconnect one more motor. Alternatively, the unused motor can be connecteddirect on load if calculations have demonstrated that this is feasiblewith respect to current peaks or other disturbances of the powertransmission system. Depending on the number of already running motorsit can be beneficial to temporarily reduce the frequency before the DOL(direct on-line) start. If necessary the power can be switched off whenstarting an additional motor and then start and ramp up the speed of allmotors simultaneously. In a compression station another option is to putall pumps and compressors in recirculation before starting up acompressor or a pump that has been stopped, then start the stopped unitand when it has reached the desired speed, put all compressors and pumpson line in production mode.

The above mentioned devices and methods make it possible to manage theFerranti effect and skin effect and thereby considerably extend thedistance for stable subsea high-voltage power transmission.

Hence maximum practical step-out distance can be very much increasedwithout introducing subsea VSDs with local subsea control of thefrequency. Both in FIGS. 4 and 5 the step-up devices have not a localcontrol system that varies the frequency and thereby the speed of motorsaccording to the production, neither do they directly control theramping down of frequency to add operation of motors that have been stopnor do they directly control the ramping up of the frequency to obtainthe actual speed of the motors to suit the production.

If the RFSD has oil lubricated bearings, there is no need for anycontrol system of the unit, and possible instrumentation can be limitedto monitoring, e.g. vibrations and temperature, if found beneficial.

As mentioned in the section: “Background of the invention and prior art”the speed of compressors can typically range from e.g. 4000 to 14000 rpmand of pumps from e.g. 2000 to 5000 rpm. When compressor and pump motorsin a compression station according to the invention (Fourth and FifthSolution) are supplied with the same frequency by a common transmissioncable, the speed of the pumps can easily be adjusted to the desiredspeed of half of the compressor speed by using four-pole or more polemotors for the pumps and two-poles

motors for the compressors. If the pumps are used for controlling theliquid level of a separator in a compressor station, a suitable variablenet forward flow for the pump can be arranged by re-circulation andequipped with flow control valves.

The speed of the pumps can therefore be controlled in the followingoptional ways:

Dedicated subsea FSD for each pump motor

One common FSD for several pumps motor

Running the pump motors on same frequency as the compressors, but withthe double number of pole resulting in half the rotational speed

Running the pumps on the transmission frequency whilst the compressorspower frequency is stepped up.

Generally, for the number of subsea FSDs, their number can be from oneper motor to one big common unit for all motors or something in between,e.g. one FSD per large compressor motor and one common unit for thequite small pump motors or, as mentioned above, no FSD for the pumpmotors.

Some Suggested Combinations of Surface Located VSDs, Number of SubseaDrives and Number of 3-Phase Transmission Line

A 3-phase transmission line consists of three individual cables that areinsulated and bundled together. For long subsea transmission with morethan one motor, e.g. two compressors, it is with present technologypossible to bundle together transmission lines for two motors, i.e. sixcables in the bundle. This will reduce the laying cost of the lines andhas the advantage of allowing individual frequency control of two motorsat the far end of the two lines that are bundled together. There is onestep-up device per motor. Such an arrangement is shown in FIG. 7. Inthis case the motor is of the high voltage type and the transmissionvoltage can be e.g. 100 kV and there is no need for subsea transformers.In such case the circuit breaker has to be located after the generatorwhere the voltage is acceptable because subsea circuit breakers for veryhigh voltages like 100 kV are presently not available.

Another way, which results in lower investment, is the solution shown inFIG. 4 and with a hydraulic soft starter between motor M and generator Gsuch that the motors M1-M4 can be started individually withoutunacceptable start currents. All motors will operate at same speed,which is not a problem for equal machines, e.g. compressors.

The less complicated arrangement is that of FIG. 4 without soft starter.In this case it will be necessary to start all compressorssimultaneously, and this is a little inconvenient but not considered aproblem because number of start-ups per year is limited.

In Table 2 is explained the meaning of the items in the figures.

TABLE 2 Figure labels. Item # Explanation 1 Electric power supply grid2, 2′, 2″, 2′″ Step-down transformer 3, 3′, 3″, 3′″ VSD, Variable SpeedDrive 4, 4′, 4″ 4′″ Step-up transformer 5, 5′, 5″, 5′″ Transmissioncable 6, 6′, 6″, 6′″ Step-down transformer 7, 7′, 7″, 7′″ Circuitbreaker 8, 8′, 8″, 78′″ Near end of transmission cable 9, 9′, 9″, 9′″Far end of transmission cable 10 Common bundle of two or more powertransmission lines 11 Pressure housing 12 Inert gas or liquid 13, 13′,13″, 13′″ Step-down transformer 14, 14″, 14″, 14′″ VSD 15, 15′, 15″,15′″ Circuit breaker 16, 16′, 16″, 16′″ Rectifier 17 Fluid (hydraulic)coupling (optional), stepless fluid gear (optional) or fixed ratio gearbox (optional) 18, 18′ Penetrator 19 Pressure balancing unit M1, M2, M3,M4 Motor M Motor of the subsea rotating frequency step-up device (subseaRFSD) G Generator of the subsea rotating frequency step-up device

DETAILED DESCRIPTION

Reference is made to FIG. 4, illustrating a specific embodiment of thepresent invention. Node 1 is connected to a source for electric power;the source is a local power grid or, for instance, a local powergeneration system. A VSD 3 is connection to power source. A VSD inputtransformer 2 is often connected in between in order to adjust thesupply voltage, e.g. 13.8 kV for a platform to the rated VSD voltage,e.g. 6 kV. The transformer can be an integrated part of the VSD asoffered by some suppliers. Normally a step up transformer 4 is needed toconnect the VSD 3 to the high-voltage transmission line 5 that in theexample of a subsea application consists of a cable. A typical voltageapplied to the cable could for instance be about 120 kV. The cable islaid into the sea in order to extend from the near end 8 to the subseafar end 9; the cable has any operative length where the Ferranti effectstarts being observed until where it strongly dominates to the loadcurrent. This can be translated to length in the order 20 km, to 100 kmand probably beyond, dictated by the location and properties of thesubsea loads. At the far end 9 of the cable, a subsea transformer 6 isarranged, stepping down the voltage to for example 20 kV suitable forthe circuit breakers 7, 7′, 7″, 7′″, followed by transformer 13, 13′,13″, 13′″ stepping down to for example 6 kV suitable for the motors ofsubsea RFSDs or the operational voltage of SFSDs, which is also asuitable voltage for the motors M1, M2, M3, M4. Four subsea motors areillustrated, which for instance could be two compressor motors M1, M2and two pump motors M3, M4.

The step down transformers are in principle optional because thestep-down transformer 6 (ref. FIGS. 4 and 5) can directly step-down thevoltage suitable for the subsea FSDs as illustrated in FIG. 5. Inclusionof 13, 13′, 13″, and 13′″ is a question of optimisation of the far endpower distribution system.

The subsea RFSDs in FIGS. 4 and 5 step up the transmission frequencywith a desired step up ratio by selection of poles of the motor M andGenerator G.

It shall be emphasised that the key components of the power transmissionsystems of FIGS. 4 and 5 are the power source 1, the variable speeddrive (VSD) 3, the transmission cable 5 and the motor-generator set M-G.The other components, i.e. step-up and step-down transformers, 2, 4, 6,and 14, 13′, 13″, 13′″, and circuit breakers 15, 7, 7′, 7″, 7′″, areincluded according to need from case to case.

If for instance the motor M of the motor-generator set is of the typewith insulated cables in the stator it can operate at a much highervoltage than motors with conventional coils. Hence both the step downtransformers 4, 6 and 13 may become superfluous. If additionally themotors M1-M4 are run at fixed sped from the step-up devices, the VSD 3can be omitted.

Another advantage of high voltage subsea motors with insulated statorcables is that they need less current (amperes) through the penetratorsthrough the motor housing than motors at conventional voltage in therange of 6 kV. This will allow for motors with higher power than at thepresent stage where around 12 MW is the maximum due the limitation incurrent (ampere) capacity.

Cost of long subsea cables and subsea VSDs is high, and subsea VSDs inFIG. 2 have a negative impact on system reliability as well as beingexpensive. One common transmission cable compared to the solution inFIG. 1 therefore represents a considerable saving in investment.

It shall be mentioned that even though one common transmission cable isbeneficial of cost reasons, there is technically no problem to have onetransmission cable for each subsea FSD. This may be the optimum solutionfor medium step out lengths, say 35 to 75 km, i.e. up to distances wherethe cable cost does not become prohibitive. With one VSD pertransmission cable, i.e. one VSD per subsea motor, this results inindividual speed control for each motor.

Condensed Description of the Invention Subsea Step-Up Device

It is problematic or even not possible to transmit high voltage highpower electricity at high frequency, say more than 100 Hz, over longsubsea step-up distances, say more than 40 km, to supply motorsoperation at high speed for subsea pumps and compressors. This is due tothe Ferranti effect that can create over voltage and instability in thetransmission system as well as the skin effect that creates high ohmicresistance and consequently high voltage and power losses.

Subsea variable speed drives to which the transmission frequency can below, e.g. 50 Hz, presents a solution to this. They are however big andequipped with a large amount of sensitive, fragile electric andelectronic components and control system, which additionally to makingthem expensive also are assumed to have a high failure rate.

The invention offers a solution to this by having the VSD with itscontrol system at surface (on a platform or onshore) and then having oneor more simple rotating subsea frequency step-up devices, near thesubsea motors. These devices preferably do not directly control thefrequency of the electric current to the motors, their only function isto step up the transmission frequency, which is variable and set atfrequency according to the need of the motors, by a suitable ratio. Inthe case of rotation subsea frequency step-up devices, the resultingstep-up ratio is resulting from the ratio of numbers of poles of thegenerator and the motor of the device. The ratio will for instance be 2if the generator is 4-pole and the motor 2-pole.

As stated above, the preferred function of a SRFSD is purely to step-upthe transmission frequency, and the variation in the output frequency isdecided by a near end surface located VSD. The exception from this is ifthere is no VSD or similar control device at the near end. In such casesthe output frequency of the SRFSD generator can be fixed or be variedwithin some limits by including some type of adjustable coupling or gear(e.g. a mechanical gear, a fluid-dynamic or hydraulic gear, a mechanicalfluid dynamic gear or a magnetic gear) between the motor and thegenerator of the SRFSD.

Rotating subsea step-up devices add inductance to the transmissionsystem and are therefore beneficial by counteracting the largecapacitance of the cable, and therefore the near end compensation systemcan probably be reduced.

The invention claimed is:
 1. A device for operative connection between asubsea step out cable far end and subsea loads such as pumps,compressors and control systems, the device comprising: a motoroperatively connected to the step out cable; a generator operativelyconnected to subsea motors of the subsea loads, wherein the motor andthe generator are operatively connected so that the motor drives thegenerator; at least one gas and/or liquid filled vessel into which atleast one of the motor and generator are arranged; wherein a length ofthe subsea step out cable is long enough to cause problems at frequencyand power levels feasible for subsea pump and compressor motors; andwherein the device is a rotating frequency step up device and isconfigured to, via the step out cable, receive input electrical power ata low enough frequency to have stable transmission and the device,operatively connected to the subsea motor, deliver an output electricalfrequency, amperage and voltage feasible for operation of the connectedmotors.
 2. The device according to claim 1, wherein the device has nomeans for active control or adjustment on site subsea, and the devicecomprises: a generator arranged on a motor shaft; a vessel into whichthe motor, generator and shaft are arranged; a liquid filling thevessel; a pressure compensator; and at least one electric penetrator. 3.The device according to claim 1, comprising an electric motor and anelectric generator having a common shaft, a pole number of the generatoris a multiple of the pole number of the motor, and the number of polesof the motor and generator is selected such that a desired frequencystep-up is achieved.
 4. The device according to claim 1, wherein apassive electric frequency stepper device comprises at least one of amechanical gear, a fluid-dynamic or hydraulic gear, a mechanical fluiddynamic gear, and a magnetic gear.
 5. The device according to claim 1,wherein a connection or coupling or shaft comprises a hydraulic or fluidcoupling.
 6. The device according to claim 5, wherein the coupling is asoft starter.
 7. The device according to claim 1, wherein a variablespeed drive (VSD) is connected at a near end to adjust a lowtransmission frequency and thereby adjust an output frequency of thegenerator up and down to give a desired speed of the connected motor ormotors.
 8. The device according to claim 1, wherein a transmissionfrequency from a power source at a near end is fixed.
 9. The deviceaccording to claim 1, wherein a housing is filled with liquid,preferably oil or a mixture of water and antifreeze agent and have apressure balancing device between ambient seawater and internal liquidof the housing.
 10. The device according to claim 1, comprising two ormore Subsea Rotating Frequency Step-up Devices (SRFSD), wherein themotors of the SRFSDs are connected to bundled transmission lines. 11.The device according to claim 1, wherein the motor of the SRFSD is of ahigh voltage type with insulated cables in a stator.
 12. The deviceaccording to claim 1, wherein: a housing is filled with gas; and thepressure inside the housing can be selected from a region of one bar upto equal to ambient water pressure or higher.
 13. The device accordingto claim 1, wherein the generator is a direct current (DC) generator.14. A system for pressure boosting of hydrocarbon fluid or other fluidsubsea, the system comprising: a subsea step out cable, connected to anelectric AC power source at a near end, a length of the subsea step outcable is too long for stable operation at frequency and power levelfeasible for subsea pressure boosting equipment; subsea motors for pumpsor compressors operatively connected to a far end of the subsea step outcable; and a rotating motor-generator frequency step up device arrangedbetween the subsea step out cable and subsea pumps or compressors. 15.The system according to claim 14, wherein the device has no means foractive control or adjustment on site subsea, and the device comprises: agenerator arranged on a motor shaft; a vessel into which the motor,generator and shaft are arranged; a liquid filling the vessel; apressure compensator; and at least one electric penetrator.